1. Field of the Invention
The present invention relates to a communication device, communication method, and computer program, operating on a stored and forward switching communication system, to perform reception processing of packets sent from other communication stations, and particularly relates to a communication device, communication method, and computer program, performing reception processing of payloads based on header signals included in packets.
More specifically, the present invention relates to a communication device, communication method, and computer program, performing payload reception operations in accordance with header error detection results, and particularly relates to a communication device, communication method, and computer program, performing accurate header error detection so as to avoid reception of useless payloads.
2. Description of the Related Art
There has been given much attention in recent years to a wireless communication format called ultra-wideband (UWB) communication, which enables high-speed transmission of 100 Mbps or faster, using an extremely wide bandwidth. For example, in the USA, the FCC (Federal Communications Commission) stipulates the spectrum mask for UWB transmission at 3.1 GHz to 10.6 GHz in an indoor environment. UWB communication is more suited for close-distance communication because of the transmission power thereof, but is capable of high-speed transmission. Accordingly, application to PAN (Personal Area Network) with a communication distance of around 10 m is envisioned, and there are expectations that UWB will be put to practical use as a wireless communication realizing close-range ultrahigh-speed transmission.
Also, there are expectations for OFDM (Orthogonal Frequency Division Multiplexing) as a technique for realizing increased speed and quality of wireless communication by avoiding deterioration in transmission quality due to fading of wireless signals. For example, the IEEE 802.15.3a standardization group has defined that DSSS (Direct Sequence Spread Spectrum)-UWB, which maximizes the spread speed of DS (Direct Spread) information signals, and also OFDM_UWB employing OFDM modulation, be employed for UWB transmission, and prototypes have been tested for each format.
Another format known for flexibly changing the frequency being used is FH (Frequency Hopping), wherein communication may be incapacitated due to effects of other systems, but communication hardly ever cuts out since the frequency is constantly being changed. That is to say, FH can coexist with other systems, has excellent anti-fading properties, and is readily scalable.
For example, the IEEE 802.15.3 standardization group is contemplating dividing the bandwidth from 3.1 GHz to 10.6 GHz set by the FCC for OFDM_UWB into multiple 528-MHz sub-bands and frequency-hopping the sub-bands, i.e., implementing OFDM_UWB as a multi-band method (hereafter referred to as “MB-OFDM”). The discussion of the IEEE 802.15.3 task group (TG3a) has been taken as an ECMA (European Computer Manufacturer Association) standard with very little change, and ECMA-368 specifies a PHY layer and MAC layer for UWB communication systems (e.g., see http://www.ecma-international.org/publications/standards/Ecma-368.htm).
Now, stored and forward switching communication systems generally employ packet communication wherein transmission data is assembled into transmission units called packets and transmitted/received between communication stations. A packet is basically configured of a preamble made up of a known training sequence, a PLCP (Physical Layer Convergence Protocol) header serving as a PHY header, and a PSDU (Physical Layer Service Data Unit) serving as a PHY payload. FIG. 4 illustrates a format example of a packet (PHY frame) in an ECMA standard. As shown in the drawing, a PLCP header includes important information such as the MAC header and PHY header.
The ECMA standards do not provide detailed stipulations regarding the configuration or behavior of receivers. Generally, receivers use the preamble to perform packet detection and synchronization detection, perform automatic gain control of low-noise amplifier, perform frequency offset correction, and so forth. Subsequently, the receiver checks the HCS (Header Check Sequence) included in the header, and receives the payload only in the event that the HCS is correct; otherwise, the payload is not received. An HCS is a signal configured of 16 bits in length for example, for determining whether or not there is error in the header.
FIG. 5 schematically illustrates the configuration of a receiver for performing packet reception processing. Also, the procedures for reception operations performed by the receiver shown in the drawing are shown in the flowchart in FIG. 6.
First, a Viterbi decoder reproduces a data stream with the maximum likelihood, i.e., with the highest probability, from data with noise included, and then in the descrambler thereafter, the data stream is descrambled (step S1). The header portion is separated out from the data stream, an HCS inspector calculates the HCS based on the PHY header and MAC header, which is matched against the HCS to be read next, thereby detecting error in the header (step S2). Only in the event that the HCS matches (Yes in step S2) is the payload received (step S3). In the event that the HCS does not match (No in step S2), the payload is not received (step S4).
However, there is the possibility that the synchronization circuit in the receiver may start reception synchronous with noise added to the reception signals instead of the preamble. In this case, the HCS is only 16 bits long, so erroneous determination may be made that the HCS is correct even though there is no information which should be received (erroneous determination occurs with a probability of ½16), and further, payload reception will start. As a result, even if signals which originally should be received arrive while receiving the payload, reception cannot be performed, so the number of times of retransmission increases, leading to deterioration in throughput.
With the frame format shown in FIG. 4, 48-bit long Reed-Solomon parity bits are transmitted in the header following the HCS. Note that Reed-Solomon code is a type of forward error-correction code whereby data error can be detected and corrected, and more specifically is a mathematical error correction method for correcting continuously-occurring errors (bursts of error), and is understood to have high error correction capabilities. Reed-Solomon code is employed with QR code, storage such as CDs, DVDs, hard disks and so forth, and communication such as ADSL and space telecommunication, for example.
While major standards relating to MB-OFDM, such as ECMA-368, state that adding the Reed-Solomon parity bits to the header at the transmitter is essential, whether or not to decode this at the receiver side is left optional.
FIG. 7 schematically illustrates the configuration of a receiver for performing packet reception processing. The procedures for reception operations performed by the receiver shown in the drawing are the same as with FIG. 6. That is, first, a Viterbi decoder reproduces a data stream with the highest probability from data with noise included. In the Reed-Solomon decoder thereafter, the Reed-Solomon parity bits are decoded, header signal error is detected, and correction thereof is performed. At the descrambler, the data stream is descrambled (step S1). The header portion is separated out from the data stream, an HCS inspector calculates the HCS based on the PHY header and MAC header, which is matched against the HCS to be read next, thereby detecting error in the header (step S2). Only in the event that the HCS matches is the payload received (step S3); in the event that the HCS does not match, the payload is not received (step S4).
The Reed-Solomon code decoding processing itself is widely-know, so description thereof will not be given here. Decoding the Reed-Solomon parity bits and detecting and correcting header signal information markedly reduces the probability of erroneous determination as compared with cases of only performing the HCS check. Accordingly, useless payload reception operations due to using only the HCS check can be avoided.
For example a proposal has been made regarding a decoding device wherein, even in the event that correct decoding could not be made at the Viterbi decoder when receiving and decoding TS (transport Stream) packets, the data can be repaired and corrected to the correct values with a Reed-Solomon decoder having correction capabilities of up to 8 bytes per TS packet (e.g., see Japanese Unexamined Patent Application Publication No. 2006-325023, paragraph 0035)